Month of birth linked to risk of MS

In the northern hemisphere, being born in May is linked to an increased risk of developing multiple sclerosis later in life, while being born in November carries the lowest risk, finds a new study published on bmj.com today.

The researchers suspect that complex interactions between genes and the environment before or shortly after birth may help to explain this link.

Their study involved 17,874 Canadian patients and 11,502 British patients with multiple sclerosis. Data on month of birth, along with detailed information on demographics and medical and family history, were collected and analysed. The comparison groups were both from the general population and from the unaffected brothers and sisters of those with MS.

In Canada, significantly fewer people with MS were born in November compared with controls. Similarly in Britain, fewer people with MS had been born in November and significantly more had been born in May. The number born in December was also significantly lower.

Adding Danish and Swedish samples to the Canadian and British results (over 42,000 people) showed a 13% increase in risk of MS for those born in May compared with November and a 19% decreased risk for those born in November compared with May.

The effect was most evident in Scotland, where the prevalence of MS is the highest.

These findings conclusively show the association between month of birth and risk of MS in northern countries, but the explanation remains unclear, say the authors.

Previous studies have suggested that exposure to the sun or seasonal variations in a mother's vitamin D levels during pregnancy may have an impact on brain development.

These findings support suggestions that environmental factors both before and immediately after birth may influence the development of the nervous or immune systems and therefore determine the risk for this disease in adult life, they conclude.

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